Several different aspects of Acanthamoeba myosins I and II are under study. Myosin I heavy chain kinase has been shown to be highly, but not fully, activated simply by binding to acidic phospholipid vesicles; maximal activation requires autophosphorylation. Autophosphorylation of vesicle-bound kinase has been shown to be both intramolecular and intervesicular and phosphorylation of vesicle-bound myosin I by vesicle- bound kinase is intervesicular. The catalytic domain of the kinase has been expressed in E. coli. Myosin IA has been shown to be 70-100% phosphorylated in situ and myosins IB and IC about 15-40% phosphorylated; membrane-associated myosins IB and IC are 10-20% phosphorylated whereas cytoplasmic IA, IB and IC are largely phosphorylated. Cortical phosphomyosins I are enriched surrounding phagocytic cups and plasma membrane-associated phosphomyosin IB is highly enriched in regions of membrane protrusions and invaginations. Electric birefringence studies demonstrate that MgATP bound to the head of minifilaments of dephosphorylated myosin II greatly increases the flexibility of the minifilaments (which are much stiffer than minifilaments of phosphorylated myosin II) but has no effect on the flexibility of minifilaments of phosphorylated myosin II. These results provide additional evidence for communication between the head and tail.